On a break from his job at the Gillette plant, Stephen Torci paused at a memorial for a killed cyclist, where a “ghost bike” stood in tribute.

He didn’t know the woman who had died in last week’s accident, an Irish native named Tanya Connolly, 37. . But amid the midday bustle of West Broadway, the lone white bike called for a moment of reflection.

“It says something about biking in Boston, doesn’t it?” Torci said Monday as he and others looked on at the streetside memorial, where ghostbikes.org mourners have left flowers, Irish flags, and handwritten condolences for Connolly.

In a grass-roots gesture of solidarity and remembrance, cyclists often place the painted bikes at the sites of fatal cycling accidents, both to mourn the victim and urge passing motorists to drive more carefully.

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For victims of car crashes and violence, street memorials are often created publicly, as part of a ceremony in which friends and family bring candles, flowers, and keepsakes. But the ghost bikes are a more mysterious custom. They appear without notice, with no indication of who is responsible.

The ritual, which seems to have begun in St. Louis in 2003, has since spread to cities around the world, and in recent years has been followed regularly in Boston.

Besides South Boston, where Connolly was struck by a tractor-trailer, a ghost bike marks the Morrissey Boulevard spot in Dorchester where Doan Bui, 63, was fatally struck by an allegedly drunken driver
on Sept. 14. A short distance away from the bike lay a bouquet of fresh flowers and an array of fruit.

The bikes, usually older models that have seen better days, are painted wheel-to-wheel in white, creating a stark, almost haunting image. They are carried to the memorial, so that no dirt smudges the tires. They are brought unnoticed, then left chained to a tree or lamppost as if their owner will soon return.

David L. Ryan/Globe Staff

On West Broadway, mourners have left a “ghost bike,” flowers, Irish flags, and handwritten notes for Tanya Connolly.

“They just appear,” said Pete Stidman, who directs the Boston Cyclists Union. “It’s a reminder to the public and to the city that we have to do something. We can’t keep letting this happen.”

A website, ghostbikes.org,
chronicles the custom with a focus on the Street Memorial Project in New York City. It describes the ghost bikes as “dignified and somber” memorials meant as a “quiet statement in support of cyclists’ right to safe travel.”

“The death of a fellow bicyclist hits home, since we travel the same unsafe streets and face the same risks; it could just as easily have been one of us,” the website states.

Carolyn Szczepanski, a spokeswoman for the League of American Bicyclists, said the roadside memorials have become a growing tradition over the past decade, at once a cautionary message and a public expression of grief.

The group estimates that at least 700 cyclists are killed in US traffic crashes each year and has embarked on a campaign to track all cycling deaths, called Every Bicyclist Counts.

Cycling advocates say Boston officials typically allow the ghost bike memorials to remain for weeks or even months before they are removed. Dot Joyce, a spokeswoman for the mayor’s office, said the city is sensitive to the memorials and the message they convey, and tries to be accommodating.

Eventually, the city reaches out to the cycling community to have the memorial taken down, as they do with other street memorials, Joyce said.

In South Boston on Monday, the sight of the freshly painted bike and a bright Irish flag drew many passersby. Most said they had never seen a memorial like this, and wondered what it meant.

One young woman, a 19-year-old named Sam, said the memorial was beautiful, in its way, but deeply sad. “I love the way they did it,” she said. “But it makes me wonder who is going to die next.”

In Dorchester, a cyclist heading toward UMass Boston slowed as she approached the ghost bike. Stephanie Politano, 27, said she had never seen one before. Told what it meant, she said she keeps to the sidewalk on her ride along the water down to Castle Island. The roads were too scary. “I hate riding in the city,” Politano said.

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